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Daggerboard - use of?

Printed From: Yachts and Yachting Online
Category: General
Forum Name: Beginner questions
Forum Discription: Advice for those who are new to sailing
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=4653
Printed Date: 14 Apr 21 at 5:51pm
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Topic: Daggerboard - use of?
Posted By: Captain Morgan
Subject: Daggerboard - use of?
Date Posted: 15 Sep 08 at 2:04pm

This might seem like a silly question, but the daggerboard/centreboard is moveable for a reason. However, after a couple of inversions where the daggerboard dropped completely out of the slot, I have started to sail my boat with the board fully down on all points of sail. I've checked my speed using GPS, and can't find much appreciable difference with the board fully down, or otherwise.

Training tells us that the board should be fully down when beating, half way up when reaching and up when running. Long established classes such as the Wayfarer (and especially Laser) for example, seem to follow these rules.

But, I've looked into this further and found that some more modern classes (e.g. RS, especially those with bungee which goes over the top of the daggerboard when it's fully down) don't seem to bother raising the daggerboard once the water is deep enough after launching. My current race training has also highlighted that the daggerboard should be lowered *before* the gybe mark - something that I've never had the nerve to do, but so far I haven't had any bad experiences!

  1. So, is gybing with the board down a bad idea?
  2. Stability on the run is improved, but as for the boat "tripping over" the board as it gybes, does this really happen?
  3. Is it really worth moving the board up and down for different points of sail? The increase in drag can't be that much, can it?

Any thoughts/suggestions please!




Replies:
Posted By: Black no sugar
Date Posted: 15 Sep 08 at 2:16pm

In a Topper, lifting the daggerboard in a reach and a run makes a big difference in light to medium winds.
In a top force 4 and above (18knts upwards), I leave the daggerboard down at all times.
Might not be the right technique, but every time I tried lifting it for the run of a gybe, I ended up pushed here and there by the waves and straight in the drink.

I'm sure you're going to have plenty of various answers depending on type of boats, usual sailing conditions and sailing skills.



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http://www.lancingsc.org.uk/index.html - Lancing SC


Posted By: ChickenTack
Date Posted: 15 Sep 08 at 2:35pm

It really does depend on the class of boat that you sail.

In a laser when its really blowing I usually leave the board 3/4 down to make it easer to keep it flat on the beat.

In anything assy i never take the centreboard up (mostly due to lack of arms/ability to keep the boat flat) but i have seen it done in RS400s

Apart from that i normally follow the guidelines



-------------
Regards,

Will

Crews union: beause its hard to be humble when you know you're so great!


Posted By: dics
Date Posted: 15 Sep 08 at 4:01pm

The 300 foils are really too big for the boat. I raise the board as the "guidelines". However, when sailing in a stiff breeze, due to the size of the board it is a must to have it up 9-12 inches on the beat as it makes sailing the boat up right a lot easier.

 



Posted By: alstorer
Date Posted: 15 Sep 08 at 4:26pm

As said, at the level where it begins to make enough of a difference to your performance, it really depends upon the boat. Skiff-type boats tend to leave it down for several reasons:

  1. Not enough time/too much sideways pressure to adjust it during bearaways and round-ups
  2. different sideways forces may require the board to be down
  3. less for crew to get caught on- not so much space on an RS800 especially once you figure in the centre main sheet and traditional kicker

as far as I can figure out, anyway. Sone do have levels to bring the board up a bit in high winds (at faster speeds, the losses of lift are offset against less adverse righting moment for overall greater efficiency) especially on flat water- in waves, there's more chance of stalling the foil.

Raising a centreboard does different things to raising a daggerboard. Raising a centreboard moves the pivot point of the boat aftwards. What this does is increase lee-helm. This helps the boat point deeper downwind more efficiently- very useful with a traditional spinnaker or the canting bow-sprit of the RS400*. Raising a daggerboard significantly (like on a Laser or Topper) does do this somewhat as well, though not to the same degree.

If you find you have a problem with the daggerboard falling out on capsizing- do you have a bit of elastic pulling it forward? On a Laser, you should have a length taught through the front of the board and round the front eye on the boat, running either side of the mast. On a Topper, there's a single line that should go round the mast and clip to itself. On both, this holds the top of the board forward in the case, meaning it shouldn't drop down when there's no side-load on it. That should stop it falling out when inverted, or at least leave enough sticking out for you to grab and pull the rest out.

As for speed- absolute speed is not the key in racing, it's what's variously reffered to as Speed to the Mark, Speed Made Good or Velocity Made Good (VMG). Two boats at the same absolute speed can have very different VMG.

I'm bound to be corrected on some technical points here, and look forward to being so. We all never stop learning.

*had a go on one last week for the first time- they point scarily deep. It's just un-natural if you're used to more conventional systems...



Posted By: feva sailor
Date Posted: 15 Sep 08 at 5:43pm

in my feva (aesametric gennaker) i dont actually go on a run for long and so i generally only go as low as a broad reach.

 

because of this i need the board down .

also as the bow is out when planing you lose the "grip" so the board keeps in contact with the water so the bow doesnt swing back on to the run



Posted By: mike ellis
Date Posted: 15 Sep 08 at 6:40pm

any RYA trining teaches you to have the board as suggested. however in the 600 i only bother to raise it on dead runs in the light stuff, otherwise there isn't enough time, and the boat gets even less stable, and its harder to bring the boat upright after you do piss it in.

As for centreplates, im not really sure about why but i know when i used to crew in a fireball when it got windy we swung the board back more for upwind to match raking the rig back i think.



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600 732, will call it Sticks and Stones when i get round to it.
Also International 14, 1318


Posted By: tmoore
Date Posted: 15 Sep 08 at 7:02pm

in high winds i actually have the board deeper in the water during the run than on a tight reach simply because it makes the boat more stable. in windy stuff then raise the board upwind in order to stop the boat 'tripping' in the gusts. in an assymetric i would never raise the board unless i was going downwind without the kite or something.

gybing with the board down is fine aslong as your timing etc is right. if your timing isnt up to it then a raised board with make it more forgiving by reducing the heeling force after the gybe. personally i always leave the board in the 300 as without it the boat is just so much more unstable....



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Landlocked in Africa
RS300 - 410
Firefly F517 - Nutshell
Micro Magic RC yacht - Eclipse


Posted By: G.R.F.
Date Posted: 15 Sep 08 at 8:27pm
I've investigated this a lot from a newcomers perspective.

Coming as I do from a world that dumps the plate totally the moment the
board planes offwind and rakes it a tad aft in windy weather to prevent
'railing' (the board tilting up on its edge like a boat heeling)

As usual depending who you ask you get conflicting advice, we ruined
half a season in the RS500 arguing over kicking the plate after an ex
scorpion helm had a go whilst i was away one weekend and immediately
kicked the plate back impressing jumanji, but he was on a two sail reach
which is fine, but the moment you put the kite up, you're best off leaving
it down, because the centre of effort shifts so far forward with the kite up
anyway.

I think alstorers explanation rings true, our Alto will go deeper with the
plate kicked back, especially with the pole canted to weather, but you do
induce lee helm.

We've yet to dump the plate totally, it's another control on the to do list,
bringing the method to do it to the helm. Usually by the time we could do
with it, we're fully wired and flying..

We knocked it back a tad on sunday upwind, investigating just how much
faster we could be sailing lower and faster up wind, it seemed to work, we
stretched our legs a fair few boat lengths from a nearby 505 that had
appeared to be catching us.


Posted By: mike ellis
Date Posted: 15 Sep 08 at 8:47pm
but how much did you fall to leeward of them GRF?

-------------
600 732, will call it Sticks and Stones when i get round to it.
Also International 14, 1318


Posted By: G.R.F.
Date Posted: 15 Sep 08 at 9:33pm
Originally posted by mike ellis

but how much did you fall to leeward of them GRF?


Well you don't actually drop too far, and it only works when you've got a big
course with wide corners, don't go trying it on a lake. You're just playing the
same sort of angles as you do downwind, shallower faster, you just have to
develop a sort of instinct for when it will pay to go wide fast and low, as
against high and narrow. We're finding new stuff out about this boat all the
time, just how many of our windsurf skills are transferable and we had a
nice long beat on Sunday and we had a bit of a lead worth risking an
experiment on with the plate.


Posted By: redback
Date Posted: 15 Sep 08 at 9:46pm
I would raise it in strong winds in the 4000 if I had
time - but we don't. Probably the sideways forces its
able to resist are related to the square of the speed
through the water so at speed a much smaller board would
be good. Also if it were not so deep the centre of
lateral resistance will be higher meaning less heeling
force.

When I used to crew a 800 it was noticeable that the
board could be stalled by over-sheeting the main in
light winds on a beat. This practically stopped the
boat and felt like sailing over an anchor warp. The
4000 is less likely to do this as the board is much
bigger and probably explains why we are faster than an
800 in the really light stuff. However I can feel a
slight difference on the helm if we are over-sheeted.

In light winds my crew sometimes lifts the board when we
are running deep - I think its a distraction and doesn't
make the slightest difference. If we were a slow boat
the slight leeway might be of benefit but in a fast boat
the board will still be gripping and so there is no
benefit. In a fast boat if it did stall and allow more
leeway the drag would be so high the speed loss would be
more of a disadvantage then the downwind distance
gained.

The 4000 has a very long pole and so if there is any
over-sheeting of the kite you get quite a lot of lee
helm - that is a disaster, so I always get the kite down
if the crew can't sail with the luff curled. I would
like to try sailing tight kite reaches in strong winds
with the board up a bit, but the loads are so high and
there's no time so it has to stay down.

Boats with pivoting boards do go upwind in a blow better
with less board since this also swings it backwards
which reduced weather helm. I remember even the Laser
benefits from a raised board in a blow since this
reduces heeling forces.

We gybe with the board right down. It probably would be
good to get it up a bit but provided you keep the boat
flat the effects of a deep board do not occur.


Posted By: JimC
Date Posted: 16 Sep 08 at 8:10am
Originally posted by redback

If we were a slow boat
the slight leeway might be of benefit


Note that this has stopped being a beginner response in any shape or form - Sorry!

I'd need to see some measured data to be convinced this isn't a myth. A board needs to be sized for the worst case situation, which is coming out of a tack at slow speed with maximum side force and minimum flow over the board. The actual amount of area you need running deep in the light is tiny by comparison.

And then consider what actually happens if the boat were to be making extra leeway, which is really just having the hull going sideways against the direction of travel. The sails have got the same apparent wind at the same angle as if the boat were going straight, they are just marginally rotated compared to each other. And extra leeway can only really be achieved by running the foil stalled or semi stalled, in which case the drag must be tremendous.

Going deep with the daggerboard up has a number of effects - the centre of lateral resistance moves aft just as it would with a pivoting centreboard, because the side force is shared with the rudder. The wetted area of the foil and thus skin friction decreases as you lift a daggerboard. The induced drag of the foil stays much the same (unlike a pivoting board where it surely must increase).

But I've come to believe that nearly everything people "know" about leeway is wrong, because the wind and the sails neither know or care in what direction the hull happens to be pointing underneath them*. Leeway is really just the angle of incidence on the centreboard...


*at least until bits of hull or rigging actually get in the way of getting the sails at the right angle...


Posted By: redback
Date Posted: 16 Sep 08 at 10:43pm
Of course I agree with JimC. Even if there were any
benefit in leeway it would be slight and not worth the
distraction it causes. However if raising the board
makes it easier to keep the boat flat then you'll find
the benefit is great. And in strong winds it is
generally a good idea. In strong winds the boat is
moving fast and so a smaller area is sufficient, also by
raising it you decrease the leverage and hence heeling
forces.

Incidentally I'm using Google Chrome now and it checks
my spelling as I go along. I like it.


Posted By: Medway Maniac
Date Posted: 17 Sep 08 at 12:05am

Originally posted by JimC


Going deep with the daggerboard up has a number of effects - the centre of lateral resistance moves aft just as it would with a pivoting centreboard, because the side force is shared with the rudder. The wetted area of the foil and thus skin friction increases. The induced drag of the foil stays much the same (unlike a pivoting board where it surely must increase).

Think you might want to edit that para, Jim ("daggerboard up.... wetted area....increases" - eh?)

But I think we agree that you don't need so much board area offwind to achieve the same leeway angle?

So one question in light-moderate weather is: assuming the helm balance is still acceptable, does the drag reduction achieved by raising the board offwind justify the distraction of raising it? I'd tentatively suggest the answer is: only if the leg being sailed is a long one and you've nothing better to do (e.g. no kite to trim).

In windy weather, Frank Bethwaite advocates raising the board whatever (though probably not that much in percentage terms) in order to raise the centre of effort to enable more thrust. But when you think that the lever arm is from a couple of metres up the mast down to half a metre below the waterline, will a few centimetres really reduce the heeling moment appreciably? Isn't it more a case of making the handling/gust response more forgiving?

I've tried it in many classes and I can't honestly say i've noticed any benefit one way or the other; but then maybe I'm insensitive to such subtleties. There was one occasion, however, when I was sailing a Laser on a reach and it was not planing; I hoiked up the board and suddenly the bow rose and the boat shot forward onto the plane (but maybe I just cleared some rubbish off the board?!)

In summary, as people have suggested, there are many factors against raising the board: - helm balance, boat stability, space in the boat, and time/hands available. On the other hand there are potential benefits of drag reduction, and boat handling and (maybe) increased thrust in a blow. Whether the pros outweigh the cons will depend on the class and the conditions. So I'd suggest that people new to a class or sailing in general look at what the top guys in their class are doing and do the same - one of them will surely have tried doing it differently and at least found there was no significant advantage.



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http://www.wilsoniansc.org.uk" rel="nofollow - Wilsonian SC
http://www.3000class.org.uk" rel="nofollow - 3000 Class


Posted By: redback
Date Posted: 17 Sep 08 at 12:17am
Ah yes that's very good advice for a beginner. Always do
what the best guys do. When you can keep up with them
then you can allow yourself to innovate.


Posted By: G.R.F.
Date Posted: 17 Sep 08 at 8:34am
Well for me this discussion is very ironical.

Years ago I designed a windsurfing board that I had built at Parkers, and
it was there that I learned about all the various types of centreboard (I
picked up on gybing plates which is another entire thread). Prior to that I had
(and was probably the first windsurf sailor to do so)converted my then racing
board to accept a proper wooden foil and that it would retract fully into
the hull. The result was nothing short of spectacular.

This discussion just simply wouldn't take place in windsurf racing circles,
any wind above a force four and you kick the plate even if you sailing
quite
high angles, you drive off the fin. (But then our rigs are raked well aft).
We can feel every nuance of the resistance of the plate, so, that reducing
its area has a positive effect in a boat, I am in no doubt. Hence my logic for
not contemplating one without a fully retracting centreboard (that and our
dodgy beach.)
Imho the reason no dinghy or skiffy appears to be able to break the 25 knot
barrier is because of the drag, if you could some how kick back your Cherub
Plates for instance and offset the lee helm by bringing the mast foot back you
would notice a huge speed increase, the drag of a plate is immense and
when it's gone the whole craft (a three metre eighty long board in our
case) leaps forward, feels light and skips over the surface.

The same used to be the case of round bottomed division 2 boards so it's
not just the planing surface.

So the point of all this. Reduce the plate surface area in the water
whenever and wherever you can would be my advice, but within MedManiacs
suggestion, only if by doing so you dont lose out in another area. (One
of the reasons for us bringing the controls back to the helm once we get
round to it).

One final point, on a board, kicking it back or reducing it a little bit never
had any great effect, it was only dumping it entirely that made the
significant difference.

The problem obviously in doing that in a boat sailed by boat handling
numpties like us...

Where are you going to stand to get it back up again in order to put the
plate back down in order to get it back up again..

Edit, just another point I recall, without the plate kicked up on a board it
is damn near impossible to bear off around a mark above a force four..In
fact prior to retracting plates they just to call it the 'force four barrier'.


Posted By: JimC
Date Posted: 17 Sep 08 at 9:27am
I can't help thinking that one of the key reasons good boards can break 25 knots so much more readily is that they aren't dumb enough to try and carry the same rag in 25 knots of wind as in 2.5 knots... Of course very immediate rig control and excellent power to weight ratios do no harm at all either.

Were it practical, which of course it isn't, it would have been an interesting comparison to have tried a vertically lifting daggerboard on your board experiments. The thing about a board that pivots in its case is that the wetted surface doesn't change very much until its almost completely up... Take this sketch as an example.



The same board is shown half and three quarters up, both as a pivoting cb and as a daggerboard. Compare the difference in areas, and also then think about the span of the foil (= length presented to the water) which is the main factor leadng to induced drag. I suspect that the one reason you had to kick the board all the way up to make much difference is that until its 90% or more of the way up there is very little difference anyway.


Posted By: Captain Morgan
Date Posted: 17 Sep 08 at 12:03pm

Thanks to all for your input, all of your comments and discussions have gone a long way to proving what I was suspecting - that there is no straightforward "method" of using the daggerboard!

To quote Medway Manic: "In summary, as people have suggested, there are many factors against raising the board: - helm balance, boat stability, space in the boat, and time/hands available. On the other hand there are potential benefits of drag reduction, and boat handling and (maybe) increased thrust in a blow. Whether the pros outweigh the cons will depend on the class and the conditions. So I'd suggest that people new to a class or sailing in general look at what the top guys in their class are doing and do the same - one of them will surely have tried doing it differently and at least found there was no significant advantage."

I would say that this is most definitely the case; RYA training teaches "the basics" in a classic way and, as sailors progress, then boat handling does become more specific. Personally, I have noticed that various boat handling techniques adopted by Class Associations are usually the preferred or even fashionable way of sailing a particular class. In at least one case, I didn't follow a Class Association guideline and subsequently found a faster, easier method of sailing the boat.

This brings me back to my initial view that maybe it's best to watch what the fastest guys are doing in their boats, but also to experiment out on the water to find whatever method works best in different conditions. Trial and error (lots of it)...



Posted By: craiggo
Date Posted: 17 Sep 08 at 12:27pm
The RS700 has a daggerboard that can be raked in the slot however to my knowledge most people arn't really bothering to use the rake function anymore. The idea was that the rig on the 700 can be raked a long way back and a second clew strap attached up the leech is used to maintain the boom at the orignal height, this is for very windy conditions. In raking the rig so far back the only way to balance the forces on paper was to reduce daggerboard area and also move the centre of effort of the board aft.

The reason for not really bothering anymore is that the sailors have worked out how to sail with the rig raked within the region of the bottom clew strap in most sailable wind conditions. This happened in the 600 where most of the travelling fleet never dropped down to the reefed rig as they learnt to handle the full rig in all practical conditions.

The 700 fleet now advocates lifting the board upto 6inches as the wind increases and you progressively de-power the sail, The lifting of the board has two effects, 1. It helps to restore the heeling moment to the normal levels, and 2. It offloads onto the rudder so that the combined centre of effort for lateral resistance moves aft as per the de-powered rig.



Posted By: mike ellis
Date Posted: 17 Sep 08 at 7:16pm

GRF:

I think one major problem with getting rid of the board on a dinghy would be that suddenly you would find you couldn't steer the boat. A board with only a fin is steered by carving on the rails, I'm not entirely convinced that the chines on a dinghy would be sufficient to provide a similar effect. I'm sure you've noticed in your sailing that tipping a boat creates steering in the opposite direction you get from tipping a board the same way, wether this is to do with hull shape or something i haven't a clue. I know that untill i worked out that I had to equate windsurfing to snowboarding not sailing in terms of steering i couldn't get the board to go the way i wanted it to go, so I'm not sure how you will steer a boat without a board using only the rudder.

I would be very interested to see the results of putting a sliding mast foot on a dinghy and trying your theory but it would be a lot of engineering. The only way i can see of doing it would be to slide mast, stays, and sheet lead positions all together, basically have a sliding frame with all the gubbins on it inside a hull to float the whole lot. If you ever do get round to it I would love to see the results.



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600 732, will call it Sticks and Stones when i get round to it.
Also International 14, 1318


Posted By: G.R.F.
Date Posted: 17 Sep 08 at 7:26pm
Originally posted by JimC

I can't help thinking that one of the key reasons good
boards can break 25 knots so much more readily is that they aren't dumb
enough to try and carry the same rag in 25 knots of wind as in 2.5
knots... Of course very immediate rig control and excellent power to
weight ratios do no harm at all either.

Were it practical, which of course it isn't, it would have been an
interesting comparison to have tried a vertically lifting daggerboard on
your board experiments. The thing about a board that pivots in its case is
that the wetted surface doesn't change very much until its almost
completely up... Take this sketch as an example.



The same board is shown half and three quarters up, both as a pivoting
cb and as a daggerboard. Compare the difference in areas, and also then
think about the span of the foil (= length presented to the water) which is
the main factor leadng to induced drag. I suspect that the one reason you
had to kick the board all the way up to make much difference is that until
its 90% or more of the way up there is very little difference anyway.


Well, at the time I did, we used to have only vertical 'dagger'boards, we
had them made by people like Phil Milanese and lots of other similar foil
builders, there were lots more in those days. They would replace the
pivoting centreboard that production boards came fitted with but were
very inefficient. We also had narrower "high wind" foils built, 'storm' swept
back foils and a variety of shapes designed to try and make those early
single slot sailboards viable in winds above 4-5, most failed.

Like vertical fins on raceboards, foils do not like the bottom of the foil
ahead of the top of the foil and this is what happens as the nose lifts in
any craft with a degree of rocker, the foil resists, keeps the nose down.

Many a 'flappy nosed' sailboard cn be controlled with a more vertical
fin/foil, as often many a 'nose down' board can often be released with a
more swept back fin. It has to be the same with boats. All the time the
foil stays in its vertical axis, every time the nose lifts, gets bumped up by
a wave or the spinnaker, the bottom of the foil will advance ahead of the
top and there will be an opposing reaction to lever itself back into the
vertcial axis.

So sweeping the plate back a bit corrects this tendency.

Then there is the forward facing surface which creates drag, sweeping it
back reduces that element.

So those are the main points for sweeping it back rather than lifting it up
although I do tend to accept the points your sketch illustrates, but it
didn't help much with sailboards.

I will dwell on the final reason sweeping back worked, and that was
steerage, as you know, sailboards are controlled by moving the rig, and
or foot weighting of the rail. With the plate down you weight the
windward rail, much as you would gybing a boat, and the fin interacts
around the plate.

Without the plate, you weight the inner rail, rather like a water skier
slalom turning and with practise in strong winds you turn fast without
dropping off the plane.

The day is yet to dawn when we can do that in the boat, we're close, but
so far we have always gone displacement before we're sheeted in on the
new tack even with a wave helping us.


Posted By: Black no sugar
Date Posted: 17 Sep 08 at 7:28pm

So, now Captain Morgan is totally confused (he won't install a sliding mast foot or apply so much amount of rake on a Wayfarer), it's time to give a quick update.

If you're sailing in a club where there are other boats of the same class, ask the good guys and do what they're doing. Once you get the hang of it, you can decide to be radical...

 

(Sorry, I'm not far from being a total beginner and I got lost by the end of page 1).



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http://www.lancingsc.org.uk/index.html - Lancing SC


Posted By: G.R.F.
Date Posted: 17 Sep 08 at 7:38pm
Personally I've never been of the opinion that because a person is labelled
'beginner' they are bereft of basic intelligence. I still consider myself a
beginner, it doesn't mean I don't want to learn everything there is to know
about my new chosen sport.

And if that means information swap and shared similar experiences, why
deny people that opportunity?

You can always skip reading the post.

If that is you're not just on the look out for something to complain about.


Posted By: NickA
Date Posted: 17 Sep 08 at 8:46pm

Doesn't a windsurfer "carve" when planing due to the rocker of the board rather than due to the lack of a centre board?  Thought you put the board up to reduce the lift rather than change the steering characteristics.

I'd love a dinghy that carved round corners like skis or a snowboard, but I guess it would need a very flat bottom, lots of rocker and the ability to come very high out the water when planing ........... or hydrofoils even.

.



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3604 ...lapse of reason
Javelin 558


Posted By: G.R.F.
Date Posted: 17 Sep 08 at 8:57pm
Originally posted by NickA

Doesn't a windsurfer "carve" when planing due to the
rocker of the board rather than due to the lack of a centre board? 

.



Well it carves due more to the introduction of the lee edge than the rocker
per se, given some boards have a straight rocker for example.

But they wouldn't carve with a plate down.

What would happen, the moment you tilted the board over, the foil would
try to climb to the surface a la foil-foil like foilers, if in fact the board was
even still planing, the plate would slow it down on the turn also.


Posted By: tmoore
Date Posted: 17 Sep 08 at 10:10pm
i think what GRF is saying is that the turning circle needed for a carve gybe is greater than the distance that occurs when gybing a boarded boat (because the boat pivots around the board). i think thats it in plain english. but i really am not sure myself

-------------
Landlocked in Africa
RS300 - 410
Firefly F517 - Nutshell
Micro Magic RC yacht - Eclipse


Posted By: Medway Maniac
Date Posted: 17 Sep 08 at 11:06pm

GRF, I'm sure the effects of introducing sweep or not into a centreboard on a sailboard are just as you claim, but your explanation of why it works doesn't fit in with any aero-, hydro-dynamic theory that I'm aware of.

The fundamental difference between the way sailboards and boats steer when the hull is canted (heeled), is surely that in the case of a dinghy the rig heels too, while in the case of a board the angle of the rig is independent of the heel of the hull.

People always go on about a dinghy hull steering a boat when it heels 'because the chines dig in' or whatever, but at least at low speeds the steering effect is entirely explicable because of the movement of the centre and line of effort of the rig in relation to the location and direction of the drag. Imho, the steering effect of a dinghy hull is very much secondary to the rig effects, at least until very high speeds when the hull forces possibly increase in significance.



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Posted By: Black no sugar
Date Posted: 17 Sep 08 at 11:30pm

Originally posted by G.R.F.

Personally I've never been of the opinion that because a person is labelled
'beginner' they are bereft of basic intelligence. I still consider myself a
beginner, it doesn't mean I don't want to learn everything there is to know
about my new chosen sport.

And if that means information swap and shared similar experiences, why
deny people that opportunity?

You can always skip reading the post.

If that is you're not just on the look out for something to complain about.

Sorry GRF, I didn't mean to sound as though I was complaining or implying that a beginner was bereft of basic intelligence. I was only trying to put the conversation into perspective and bring an easy solution to Captain Morgan's puzzlement when he's next on the water.

All this debate about friction and wetted area etc. is nothing short of fascinating but I was just being practical. If someone's lacking intelligence, it must be me. Spare me the acerbic comments, I won't understand them anyway.   



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http://www.lancingsc.org.uk/index.html - Lancing SC


Posted By: G.R.F.
Date Posted: 17 Sep 08 at 11:59pm
Originally posted by Medway Maniac

GRF, I'm sure the effects of introducing
sweep or not into a centreboard on a sailboard are just as you claim, but
your explanation of why it works doesn't fit in with any aero-, hydro-
dynamic theory that I'm aware of.


The fundamental difference between the way sailboards and boats
steer when the hull is canted (heeled), is surely that in the case of a
dinghy the rig heels too, while in the case of a board the angle of the rig
is independent of the heel of the hull.


People always go on about a dinghy hull steering a boat when it heels
'because the chines dig in' or whatever, but at least at low speeds the
steering effect is entirely explicable because of the movement of the
centre and line of effort of the rig in relation to the location and direction
of the drag. Imho, the steering effect of a dinghy hull is very much
secondary to the rig effects, at least until very high speeds when the hull
forces possibly increase in significance.



Well BNS is right in so far this is a beginner thread and although I have
been trying to illuminate in generalities, once you start on the Aero &
Hydro dynamic theory and a lot of it is still theory, from what I can
deduce from the countless hours of study in this internet place of joy,
vying Bernouilli with Attack Angle thinking, it begins to get complicated.

All i can say with a degree of practically tested veracity and you'll just
have to trust me on this, is that foils need their foil to operate within the
fluid they operate, be that air or water with the tip parallel to or slightly
aft of the head and they will fight hard to remain that way.

Put the tip ahead of the head and they become prone to stall, try it some
time, if you can jiggle your plate so the bottom is further forward than
the top, they don't like it.

As to steering dinghy's, I'm no expert even with the stick thing they give
you that jams against the hull or gets tangled up with rope when you
need it most, so I bow to your greater knowledge although i can't say I've
noticed any great 'hull steering effect'.


Posted By: tmoore
Date Posted: 18 Sep 08 at 12:59am

GRF if the boards stall when the bottom of the plate is further forewards than the top, why is it that all GP14's sail upwind with the board canted forewards? that is their 'fully down' position.

by the way this is an honest question, im sure someone can present a suitable answer as to why this is different from windsurfers (are they different?) the only thing i can think of is that the boat balances better this way. but then if what GRF says is true surely it would have been so much better to simply move the whole board and slot a few cm further forewards in the boat?



-------------
Landlocked in Africa
RS300 - 410
Firefly F517 - Nutshell
Micro Magic RC yacht - Eclipse


Posted By: alstorer
Date Posted: 18 Sep 08 at 7:26am
tmoore- once boat trim is taken into account, is the GP board really (on a properly trimmed boat) past vertical?


Posted By: G.R.F.
Date Posted: 18 Sep 08 at 9:37am
Well, I'm not a GP14 sailor (perish the thought, I'd hate to hear of shop
staff cowering in the backroom rather than serve me ) But my guess
would be they only do it in light wind and if they did pursue this course
beyond light wind, the effect would be for the boat to appear to point
high, but in fact make excess leeway.

Now this also depends on the type of foil employed here, I knew this
would get complicated. Foils react very differently depending on the type
they are, I'll try and stay lucid. Slow foils, which are quite fat and have the
highest point about 30% back from the leading edge, as against fast foils,
which are thinner and have the highest point about 40% back.

My guess, again I've never even been close to a GP14, but my guess
would be it's a slower foil. Slower foils can be abused a bit more, and
now I'm going to have to address a bit of 'Fuller' theory here, you have
probably read about the principle of foil dynamics, lift created on the de
pressurised edge according to Bernoulli's principle and that lift is
proportionate to the speed of the flow or the depth of the foil to induce
that pressure difference (Big Heavy lift aircraft have fat wings, Faster jet
aircraft have thinner wings and require more airspeed over the surface)

Well personally and I might be entirely wrong here, I think the lift
generated by the Bernoulli thing is only finite and most of the real lift
comes from the Angle of Attack, and my belief on slow foils is that they
give a greater range of attack angles, bigger the entry, bigger the foil,
bigger the attack angle, and more important to sailing dinghies, wider the
range for windward performance.

So in essence you can bugger about more with a 'slow foil' than you can
with a fast foil is that condensed down to explain it.

Back again to early days of windsurfing, if we wanted to 'stuff' it in light
winds, we would do just that, let the tip go ahead of the head, and the
boards would point like crazy, but, once the pressure gets to a certain
level, they began to make massive leeway and you had to centre them and
foot off a bit or you would simply die. Now this all got trialled in a one
design environment, so I know it to be true, but, as I keep reminding
y'all, I only know it to be true of windsurfing boards, it's just my
assumption that dinghies must be the same, only you wouldn't be quite
as aware because you can't feel the pressures so much through your arms
and legs as we do.

Sorry if that got a bit technical and tedious, I've researched it a lot over
the years, you won't believe the lengths windsurfers go to over fins, 100's
of hours in German Test Tanks, and sails and kites in wind tunnels in the
States, we can get really anal about it. Did you also know, you should in
theory match the foil size to your sail size to achieve maximum
efficiency?

It's why we get very dismissive about so called 'skiff' efficiency, when in
fact they are very inefficient.


Posted By: craiggo
Date Posted: 18 Sep 08 at 10:08am
Forward swept foils work however the foil has to be reasonably stiff.

The problem with forward sweep is that it puts the centre of effort ahead of the attachment, as a result when it sees an angle of attack, rather than weathercock into the flow it will do the opposite and diverge. If the foil isnt strong enough it will twist itself apart unless you can make your foil stiff enough to counter this problem, then it works brilliantly. If your foil starts twisting off, rather than seing 3degrees of AoA it will twist maybe 5degrees and will then see 8degrees AoA which may well lead to seperation and a stall.
If the foil is too stiff though it wont depower naturally so will require more skill to sail with.

Sorry for being too technical.

Paul


Posted By: G.R.F.
Date Posted: 18 Sep 08 at 10:52am
Originally posted by craiggo

Forward swept foils work however the foil has to be
reasonably stiff.

The problem with forward sweep is that it puts the centre of effort ahead
of the attachment, as a result when it sees an angle of attack, rather than
weathercock into the flow it will do the opposite and diverge. If the foil
isnt strong enough it will twist itself apart unless you can make your foil
stiff enough to counter this problem, then it works brilliantly. If your foil
starts twisting off, rather than seing 3degrees of AoA it will twist maybe
5degrees and will then see 8degrees AoA which may well lead to
seperation and a stall.
If the foil is too stiff though it wont depower naturally so will require
more skill to sail with.

Sorry for being too technical.

Paul


Agree with everything said there, and to further qualify my 'statements'
I'm obviously talking about foils designed to work in the vertical axis,
obviously foils can be designed to to work in any plane.

As a rider to all this, no-one should get too excited about it, and judging
by a lot of the dinghy foils I've come across, they're pretty agricultural
devices so will take a deal of abuse.

But they do remain a source of resistance to forward movement off wind
and the slower the foil shape the greater the resistance so in theory the
more you might gain by lifting or tilting it back downwind, remains my
considered opinion. (And JimC's sketch remains also true)


Posted By: tgruitt
Date Posted: 18 Sep 08 at 11:16am
Originally posted by G.R.F.

Well, I'm not a GP14 sailor (perish the thought, I'd hate
to hear of shop
staff cowering in the backroom rather than serve me ) But my guess
would be they only do it in light wind and if they did pursue this course
beyond light wind, the effect would be for the boat to appear to point
high, but in fact make excess leeway.

Now this also depends on the type of foil employed here, I knew this
would get complicated. Foils react very differently depending on the type
they are, I'll try and stay lucid. Slow foils, which are quite fat and have the
highest point about 30% back from the leading edge, as against fast foils,
which are thinner and have the highest point about 40% back.

My guess, again I've never even been close to a GP14, but my guess
would be it's a slower foil. Slower foils can be abused a bit more, and
now I'm going to have to address a bit of 'Fuller' theory here, you have
probably read about the principle of foil dynamics, lift created on the de
pressurised edge according to Bernoulli's principle and that lift is
proportionate to the speed of the flow or the depth of the foil to induce
that pressure difference (Big Heavy lift aircraft have fat wings, Faster jet
aircraft have thinner wings and require more airspeed over the surface)

Well personally and I might be entirely wrong here, I think the lift
generated by the Bernoulli thing is only finite and most of the real lift
comes from the Angle of Attack, and my belief on slow foils is that they
give a greater range of attack angles, bigger the entry, bigger the foil,
bigger the attack angle, and more important to sailing dinghies, wider the
range for windward performance.

So in essence you can bugger about more with a 'slow foil' than you can
with a fast foil is that condensed down to explain it.

Back again to early days of windsurfing, if we wanted to 'stuff' it in light
winds, we would do just that, let the tip go ahead of the head, and the
boards would point like crazy, but, once the pressure gets to a certain
level, they began to make massive leeway and you had to centre them and
foot off a bit or you would simply die. Now this all got trialled in a one
design environment, so I know it to be true, but, as I keep reminding
y'all, I only know it to be true of windsurfing boards, it's just my
assumption that dinghies must be the same, only you wouldn't be quite
as aware because you can't feel the pressures so much through your arms
and legs as we do.

Sorry if that got a bit technical and tedious, I've researched it a lot over
the years, you won't believe the lengths windsurfers go to over fins, 100's
of hours in German Test Tanks, and sails and kites in wind tunnels in the
States, we can get really anal about it. Did you also know, you should in
theory match the foil size to your sail size to achieve maximum
efficiency?

It's why we get very dismissive about so called 'skiff' efficiency, when in
fact they are very inefficient.


Agreed. Fantastically put, couldn't explain it any better. On the N12 I
used to sail, we would ram the centreboard right down so that the tip was
in front of vertical, great in light winds, and as you say, soon as the
pressure build, a lot of sidewaysness happened.


-------------
Needs to sail more...


Posted By: Merlinboy
Date Posted: 18 Sep 08 at 4:51pm
Its true we sail with the board slightly forward in the light stuff, glad to know i'm at least doing this right.

-------------


Posted By: JimC
Date Posted: 18 Sep 08 at 5:28pm
Originally posted by G.R.F.

Did you also know, you should in
theory match the foil size to your sail size to achieve maximum efficiency?


Only for the last 40 years... Its hardly a new discovery. I'm sure I've seen it discussed in books from the 1930s...

The thing about raked forward foils is only really to do with those that will twist... If a foil twists significantly under load then the angle of incidence will similarly vary, and of course in opposite directions depending on which way it twists, which in turn is related to whether its raked forward or aft (amongst other factors like subtleties of construction).

To extent to which this twist is desirable depends on a lot of other factors, rather like the interminable conversations on how/if gybing daggerboards work. A forward raked twisting board is, after all, gybing at the tip, but not the root... But maybe we should ask for this thread to be moved to development...


Posted By: mike ellis
Date Posted: 18 Sep 08 at 6:42pm

Originally posted by G.R.F.

As to steering dinghy's, I'm no expert even with the stick thing they give
you that jams against the hull or gets tangled up with rope when you
need it most, so I bow to your greater knowledge although i can't say I've
noticed any great 'hull steering effect'.

the best way to demonstrate hull steering on a dinghy is to try to bear away with the boat heeled to leeward (away from you, then try the same thing with the boat heeled to windward with all the controls the same. Unless something very wierd happens with the Alto you should notice that it is significantly easier when heeled on top of you.

This is what i was on about with windsurfers using "hull" steering the opposite way to dinghies.



-------------
600 732, will call it Sticks and Stones when i get round to it.
Also International 14, 1318


Posted By: Medway Maniac
Date Posted: 18 Sep 08 at 8:58pm
I dispute whether that has much to do with 'hull steering', Mike. There may be a small effect, but the steering to which you're referring is totally explicable by the movement of the line of action of the sails one side then the other of the line of action of the hull resistance. If you could cant the rig while keeping the hull flat, you'd still get the same effect.

-------------
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Posted By: G.R.F.
Date Posted: 18 Sep 08 at 10:34pm
Originally posted by mike ellis

[

the best way to demonstrate hull steering on a dinghy is to try to bear
away with the boat heeled to leeward (away from you, then try the same
thing with the boat heeled to windward with all the controls the same.
Unless something very wierd happens with the Alto you should notice that it
is significantly easier when heeled on top of you.


This is what i was on about with windsurfers using "hull" steering the
opposite way to dinghies.



Well that's exactly what happens when you gybe a windsurfer with its plate
down, you weight the windward rail. I'd heard it works on dinghies, I shall
try it a little harder now on the bear off and round the gybe.


Posted By: Jack Sparrow
Date Posted: 19 Sep 08 at 8:41am
as for foil sweep this makes interesting reading... http://speedsailingdesign.blogspot.com/2007/01/tutorial-article-on-keel-design-methods.html - link

and here... http://www.culnane.net/dc/sailing/moth/rake/index.html - link


-------------
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Posted By: redback
Date Posted: 19 Sep 08 at 11:01pm
In GP14s and Scorpions (of which I have more experience) the centreboard is moved forward of vertical to give a bit of feel to the rudder which makes it easier to sail upwind in light winds.  As soon as the wind fills in the weather helm generated is a bit too much and the board is moved to vertical.


Posted By: G.R.F.
Date Posted: 20 Sep 08 at 10:20am
So then we're all agreed now are we?

That when the bottom of the foil goes ahead of the top, in 'filled in' wind,
it acts in such a way that to continue efficiently it needs to be moved
aft..?

Yes?


So how do we now feel about planing skiffs with fixed foils when the nose
is lifted by the Spinnaker and or a wave and that action has the very same
effect?

Which is the point I was trying to make, and why skiffs in their present
incarnation are inefficient and will, like foiling moths, always have a
'speed ceiling' created by that inefficiency amongst others..(Like stern
drag or 'tail walking')

Interesting looking at things from a windsurfers perspective isn't it?


Posted By: redback
Date Posted: 20 Sep 08 at 5:10pm
You are right - ideally the board should sweep back and
even better reduce in area but at high speed we'd not
want all the turbulence caused by a centreboard case, so
we'll have to stick to a dagger. I really must remember
to get the board up a bit in a blow it might help reduce
that horrible twitchy feeling when we're doing over
15knots.


Posted By: dopamine
Date Posted: 20 Sep 08 at 7:04pm
Originally posted by redback

You are right - ideally the board should sweep back and
even better reduce in area but at high speed we'd not
want all the turbulence caused by a centreboard case, so
we'll have to stick to a dagger. I really must remember
to get the board up a bit in a blow it might help reduce
that horrible twitchy feeling when we're doing over
15knots.


But, in my limited experience, the 4k really lifts its nose when the kite powers up. The RS800 sails much flatter and you can sense it wanting to pitchpole you.

Looks like the Alto pole is quite high, lifting the bow?


Posted By: alstorer
Date Posted: 20 Sep 08 at 7:54pm
Originally posted by G.R.F.

So then we're all agreed now are we?

That when the bottom of the foil goes ahead of the top, in 'filled in' wind,
it acts in such a way that to continue efficiently it needs to be moved
aft..?

Yes?


So how do we now feel about planing skiffs with fixed foils when the nose
is lifted by the Spinnaker and or a wave and that action has the very same
effect?

Which is the point I was trying to make, and why skiffs in their present
incarnation are inefficient and will, like foiling moths, always have a
'speed ceiling' created by that inefficiency amongst others..(Like stern
drag or 'tail walking')

Interesting looking at things from a windsurfers perspective isn't it?


There's many trade offs to be had in boat design- that's why the 50knot mark is being chased by windsurfers, wierd solid wing one-tack specials and a massive foiling trimaran..

And whilst the Moth and Cherub types might be able to make use of this perspective, those of us on stricter classes just have to learn to live with what we've got.

But yes, interesting. As ever, no definitive answer.


Posted By: JimC
Date Posted: 20 Sep 08 at 8:44pm
Originally posted by G.R.F.

So then we're all agreed now are we?
Yes?

No. Its an over simplification which relates to certain specific circumstances, most especially an over flexible foil.

Originally posted by G.R.F.

why skiffs in their present
incarnation are inefficient


Oh good grief, here we go again... Skiffs are course racing boats. They're not intended to hit top speed on a limited number of directions only, so amazingly enough they're not as good at it as craft that are...

And the fastest course racing boat in the World are probably the solid wing sail C Class Cats, and according to the guys that sail them they struggle to get much past 20 knots... However they sail close to that *all the time* at amazing apparent wind angles... And in some repects, notably speed achieved as a multiples of wind strength, they are some of the most efficient water craft of all...

One of the dimmer witted statements you'll hear round the net is that boards aren't very efficient because whilst they can get damn close to 50knots, and will probably get there very soon (possibly beaten by the kiters), they can only do it is shedloads of wind. These folk claim that multihulls which can't handle the shedloads of wind but can do three times wind speed as opposed to not much more than one times are far more efficient than boards. Frankly that's ****, and I suspect you would agree.

Now extrapolate the logic...



Posted By: G.R.F.
Date Posted: 20 Sep 08 at 8:45pm
50 knots has been broken this week or it was then they downgraded it
apparently
http://www.luderitz-speed.com/ - Kites


Posted By: Black no sugar
Date Posted: 20 Sep 08 at 8:46pm
Yeah but can you take your sandwiches on board?

-------------
http://www.lancingsc.org.uk/index.html - Lancing SC


Posted By: G.R.F.
Date Posted: 20 Sep 08 at 9:05pm
Originally posted by JimC

   These folk claim that multihulls which can't handle the
shedloads of wind but can do three times wind speed as opposed to not
much more than one times are far more efficient than boards. Frankly
that's ****, and I suspect you would agree.

Now extrapolate the logic...



Well as it happens, I've got a part built mini cat in the shed right now, I'm
hoping to improve on the efficiency ratio a bit further up the wind scale.

Now you're entering a whole world of theory of how to go fast(er)

Windsurfers can get twice windspeed, they have their moments..

But having come quite close to Cats in that round island race, I have to
accept they're fast, it's down to the wide footprint to hold the (big)rig up
there, and the well reduced wetted area particularly when they fly a hull.

Now this is going right off topic, I'm in the closing stages of my speed cat
board thing, I'll bung some pics up when it's finished, I'm also trying to
get a special paint surface made from a powdered hydrophillic compound
to coat the under hull surface with.

So we could do the Cat discussion on that thread as and when I start it.


Posted By: redback
Date Posted: 22 Sep 08 at 10:56pm

Just getting back to the part of the topic about moving the board back.  An alternative is to move the rig forward!  The 4000 as with most skiffs do this by putting up another sail - often at least as big again and all forward of the mast.

 

With the 4000 its easy to get lee-helm so you wouldn't want to bring the board back any further.



Posted By: Granite
Date Posted: 22 Sep 08 at 11:13pm

Originally posted by G.R.F.

So then we're all agreed now are we?

That when the bottom of the foil goes ahead of the top, in 'filled in' wind,
it acts in such a way that to continue efficiently it needs to be moved
aft..? 

From my (basic) understanding of aerodynamics I cant think how sweeping the foil forward by 5 degrees would be significantly different to sweeping it aft by 5 degrees.

With the aft swept foil there is likely to be slightly more spanwise flow but this would make the foil less efficient. The spanwise flow contribution from the sweep is likely to be small compared to that from the pressure differential at the tip of the foil.

Sweep on aircraft is usually about reducing the drag at transonic speeds.

Forward swept wings on aircraft would be equally good at reducing the the drag at transonic speeds but due to flexing can cause all sorts of control problems, partiularly as the ailerons are at the tips and can cause the tips to flex even more leading to control reversal.

I think the problem seen is more to do with the forward raked foil twisting, increasing angle of attack and stalling. If you could build it stiff enough I do not think this would happen.

What were your windurf fins made from?

 



-------------
If it doesn't break it's too heavy; if it does it wasn't built right


Posted By: G.R.F.
Date Posted: 22 Sep 08 at 11:26pm
Windsurf fins, at least the sort that would make a difference were
generally carbon, or milled G10, a hi modulus super dimensionally stable
compound.

The sort of fin that has the effect I'm speaking of, were called blade fins, tall
hi aspect foils, generally fast profiles, although there were experiments
with slower profiles merging into negative almost reverse foil to induce lateral
resistance, but maximising the leverage for and aft, windward performance
and attack angles over and extremely narrow operating shaft.

Difficult to describe, I need pics but it was a while ago and i can't find any
on tinternet.

Anyway these fins were used to actually describe the planing angle the
board would attain.

Imagine the foil of the boat being designed in such a way that it would
resist stern drag, or nose dives by controlling the boats planing attitude.
I guess in the way those foilers must work, but without the horizontal
axis they use.

Probably unlikely it could be achieved given the greater weight of a boat
over a board and thus greater forces.


Posted By: redback
Date Posted: 22 Sep 08 at 11:47pm

I guess you've seen the "T" foil rudders used on some I14 and Cherubs. 

As for rake angle - I have experimented with this myself.  Its not so good with a forward raked foil.  I have no evidence for this but it didn't feel good if it stalled.  I surmise that when the foil stalls the turbulence starts at the tip and works up the foil.  I imagined the turbulence was "washed" up the foil if the rake was forward.  If however the foil was raked aft the turbulence was "washed" downwards and so it didn't stall so suddenly and the stall seemd to be released quickly if the angle of attack was quickly reduced.




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